This invention pertains to photographic developers, and in particular to novel photographic developer precursors in the form of solid particles in a dispersion.
Many photographic product and process improvements and beneficial new photographic products and processes could be made available with the introduction of suitable silver halide color forming developers, which can be dispersed with the silver halide at the time of manufacture of the respective photographic element. It is well known in the art that the introduction of conventional color developers, such as p-aminophenols, into sensitized photographic elements containing silver halide salts, leads to desensitization of the silver halide emulsion and unsuitable fog in such layers. Much effort has therefore been directed at trying to produce effective blocked developers, which can be introduced in silver halide emulsion elements without deleterious desensitization or fog effects and which unblock chemically under conditions of development so that developer is free to participate in color forming (dye forming) reactions.
U.S. Pat. No. 3,342,599, to Reeves, discloses the use of Schiff base developer precursors. Schleigh and Faul, in a Research Disclosure (129 (1975) pp. 27-30), described the quaternary blocking of color developer and the acetamido blocking of p-phenylenediamines. Subsequently, U.S. Pat. No. 4,157,915, to Hamaoka et al., and U.S. Pat. No. 4,060,418, to Waxman and Mourning, describe the preparation and use of carbamate blocked p-phenylenediamines.
All of these approaches and inventions have failed in practical product applications because of one or more of the following problems: desensitization of sensitized silver halide; unacceptably slow unblocking kinetics; thermal instability of blocked developer yielding increased fog and/or decreased Dmax after storage.
Recent developments in blocking and switching chemistry have led to blocked p-phenylenediamines that perform reasonably well. In particular, compounds having ".beta.-ketoester" (strictly, .beta.-ketoacyl) blocking groups are described in U.S. Pat. No. 5,019,492. The incorporation of blocked developers in photographic elements is carried out using colloidal gelatin dispersions of the blocked developers. These dispersions are prepared using means well known in the art, wherein the developer precursor is dissolved in a high vapor pressure organic solvent (for example, ethyl acetate), along with, in some cases, a low vapor pressure organic solvent (such as dibutylphthalate), and then emulsified with an aqueous surfactant gelatin solution. After emulsification, usually done with a colloid mill, the high vapor pressure organic solvent is removed by evaporation or by washing, as is well known in the art.
Certain examples of these new classes of blocked developers exhibit one or more of the following problems, however. For instance, during emulsification, the developer precursor can crystallize and phase-separate from the oil phase of the emulsion, where such crystallites are sufficiently large (greater than 1 .mu.m in largest dimension) to give deleterious coating properties and insufficient dye-forming activity. After emulsification during removal of the high vapor pressure organic solvent, the developer precursors can crystallize, undergoing a phase transformation, leading to large crystallites (greater than 1 .mu.m in largest dimension), poor coating quality, and poor unblocking and dye-forming activity. After the developer precursor dispersions are coated in a photographic element, upon storage at moderate humidity and temperature, or at high humidity and temperature, the developer precursors can aggregate and form large crystallites (greater than 1 .mu.m in largest dimension) by molecularly diffusing through the gelatin binder, leading to unacceptably low unblocking activity. During the emulsification procedure, at the elevated temperatures commonly encountered, partial thermal decomposition of the developer precursor can occur, leading to wasteful destruction of the developer precursors, to decreased dye-forming activity, and to deleterious sensitization effects on the silver halide emulsion. Finally, after coating in a sensitized silver halide photographic element, and during storage at moderate humidity and temperature or at elevated humidity and temperature, partial thermal decomposition of the developer precursor can occur, leading to loss of precursor and subsequently to a loss in dye-forming activity, and to deleterious sensitization effects from the decomposition products.
Processes that produce submicron solid particle colloids have found advantageous use in producing dispersions of photographically useful chemicals such as visible-region filter dyes, sensitizing dyes, etc. U.S. Pat. Nos. 4,294,916 and 4,294,917, to Postle et al., describe solid dispersions of photographic filter dyes and offer the advantage that such dyes in the solid state give broad spectral absorption. The preparation and use of apparently solid particle dispersions of spectrally sensitizing dyes is disclosed in German Patent No. 1,547,705. A process for producing solid particle dispersions using sand, bead, dyno, and Masap mills, and using mills described in U.S. Pat. Nos. 2,581,414 and 2,855,156, is disclosed in British Patent No. 1,570,362. The inventors of the British patent state that non-diffusing, slightly soluble color couplers can be dispersed by the milling process described. DIR (development inhibitor releasing) couplers, spectral sensitizing dyes, and photographic stabilizers are examples of other photographically useful materials that the British patent states may be dispersed as solid particle milled dispersions. There is, however, no teaching available to indicate that photographic color-forming developers may be dispersed as solid particle dispersions.
A need has existed for a photographic developer that avoids the problems described above. In particular, it would be desirable to provide such a developer in the form of a dispersion. It would also be desirable to provide a method for developing an image using the dispersion, and a photographic element including the dispersion.